Back in July we reported on a new theory explaining why both Pioneers 10 and 11 were decelerating at a rate that seemed to defy Newtonian physics. The answer, it seemed, had to do with heat from the electrical subsystems and the decay of plutonium which was pushing back on the craft. But now, a researcher from the University of Missouri says this is wrong — and that our unexpected measurements of the Pioneer probes can be explained by taking the ongoing expansion of the universe into account.

It was a mystery that threatened to overturn over a hundred years of scientific canon. How was it…
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According to Sergei Kopeikin, the previous explanation for the so-called Pioneer anomaly was only able to account for 15 to 20% of the observed deceleration. Kopeikin, on the other hand, devised a new set of calculations which factored in the expansion of the universe — including the way it affects the movement of photons that make up light and radio waves.

In order to measure the speed of the spacecraft, NASA scientists transmitted beams of radio waves to the object, and waited for their return to Earth after bouncing back (what's called Doppler-tracking). The speed of the probes could thus be determined by measuring the time it took for the photons to make a complete round trip. But what Kopeikin observed was that the phontons were moving at a different rate than predicted by Newtonian theory — what gave the impression of deceleration.

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In other words, the Pioneer spacecrafts aren't slowing down — they're moving exactly as the physical laws would predict. But because space is expanding, and because the Pioneer probes are so far away, we've been getting the false sense that they're slowing down. Physicists, it now appears, haven't been plugging in all the relevant variables into their calculations.

Consequently, Kopeikin's discovery (and his new calculations) will change the way that physicists measure the speed of objects at extreme distances — including possible interstellar trips made by future humans.